The present invention relates in general to communication systems, and is particularly directed to a new and improved polarization diversity-based signal processing mechanism for separating signals transmitted from multiple sources in a substantially flat fading environment and received at generally co-located diversely polarized antennas of a wireless communication base station.
The continuing growth of wireless communication services has generated a demand for signal usage and processing techniques that can increase the capacity of the limited cellular spectrum. In many systems, spectrum availability has reached the saturation point, so that it is not possible to acquire more frequencies; as a result, to accommodate future growth, existing frequencies must be shared among more users. This, in turn, implies the need for more sophisticated signal processing schemes to separate the signals at the receiver (e.g., base station).
A variety of interference avoidance schemes are currently used for separating users sharing the same frequency. Some methods, such as time division and code division techniques, require coordination and cooperation among users. However, not all methods require user cooperation; some rely on enhanced signal processing at the receiver. For example, advanced or xe2x80x98smartxe2x80x99 signal separation techniques that use an array of antenna elements are able to increase user capacity, by controllably shaping the array""s radiation pattern to separate users transmitting from different angular locations.
As illustrated in the reduced complexity wireless system diagram of FIG. 1, these systems employ the beam-forming ability of antenna arrays at a base station 11 to increase the antenna gain of a main antenna beam 12 in the direction of the desired user 13, while at the same time selectively decreasing the antenna gain of sidelobe beams 14 in the direction of interfering signals 15. For most applications, enhancing the desired signal of interest and suppressing the interference makes smart antennas highly effective in enabling more users to share the same frequency or channel.
However, because they rely on directional information, such beam-forming antennas cannot separate multiple signals (as in the case of high user density), in which the desired signal and one or more interfering signals originate from essentially the same (or generally co-linear) direction or angle of arrival, as shown diagrammatically at 13 and 16, respectively in FIG. 2. To prevent such co-linear interference, it is necessary to employ complicated spectrum management schemes, which may render the resulting antenna system prohibitively expensive.
One proposed enhancement to beam-forming schemes is to have the base station receiver rely on multipath differences between each user. This method has the advantage that it can be used with other methods and does not require cooperation among users. Rather than rely on transmission differences (e.g., time, angle, code) between users, the multipath processing approach relies on the different propagation environment seen by each user. A fundamental drawback to this approach is the fact that it is based on non-linear minimization, and is very complex to implement.
For many cellular installations, the base station is located on a tower, so that multipath arrival is due principally to reflections from objects that are very near to the mobile transmitter. Since these reflections impart nearly the same path delay, the multipath delay spread among plural arriving signals may be less than a sample interval. When this occurs, the multipath environment may be considered to be a substantially flat fading environment.
Pursuant to the present invention, the above-discussed substantially co-linear interference problem is successfully addressed by exploiting characteristics of the incoming signals other than their direction of arrival. In particular, the signal separation scheme of the present invention is based on multipath differences that arise in a substantially flat fading environment, when signals arrive at at least two (base station) antenna elements having diverse characteristics from the same or substantially the same direction passing through a common lobe of a radiation pattern of the base station""s antenna.
In accordance with a preferred embodiment, the invention exploits diversity gained through the use of plural (e.g., a pair of) antenna array elements having different (e.g., mutually orthogonal) polarizations and also generally co-located to reduce hardware complexity. When deployed in an environment that does not impose time-dispersion (i.e., one that has substantially flat fading), the polarization diversity-based separation scheme of the invention provides an extremely simple technique for separating co-linear signals. This, in turn, enables a smart antenna system to operate without complicated spectral management techniques.
Like existing multipath processing schemes, the invention can be used in conjunction with multiple access waveforms (e.g., time-division or code-division) and can complement other signal separation methods such as beam-forming, referenced above. The signal separation method of the invention also does not require cooperation among users, but rather relies on environmental differences between each user to emphasize one user over the other at one of the respectively diverse characteristic antenna elements, and to emphasize the other user over the one at the other antenna element.
While the invention can be used with antenna elements that are spatially separated by some prescribed distance, it is primarily intended for the case of generally co-located antenna elements that are designed to receive different (relatively orthogonal, e.g., vertical and horizontal) polarizations, and thereby reduces the size of the receiver antenna. Moreover, unlike other multipath methods, the invention employs linear signal processing, which reduces the complexity required to separate two potentially interfering signals.
In a preferred, but non-limiting embodiment, the polarization diversity-based signal separation receiver architecture of the a invention comprise four signal processing units: an RF downconverter, a signal separator, a coefficient emulator, and a channel estimator. The RF downconverter provides the signal separator with baseband, discrete-time samples of signal waveforms, that are received at a plurality of one or more pairs of antenna elements, having respectively different sensitivity characteristics. As pointed out above, in a non-limiting, but preferred embodiment, the antenna elements of each pair are generally co-located and are mutually orthogonally polarized.
The signals received by orthogonally polarized antenna elements are downconverted to baseband, and then filtered in low pass filters to remove vestigial sideband images and limit the bandwidth. The filtered baseband signals are digitized and coupled to respective inputs of the signal separator, which controllably weights and combines the baseband, discrete-time sample signals of the vertically and horizontally polarized received signals to estimate which signals emanate from which users.
For this purpose, the signal separator multiplies each signal sample received by the vertically polarized, antenna by first and second vertical polarization coefficients supplied by the coefficient calculator. It also multiplies each signal sample received by xe2x80x98horizontally polarizedxe2x80x99 antenna by first and second horizontal polarization coefficients supplied by the coefficient calculator. The products are summed in pairs to produce weighted and combined output signals that are output as first and second separated signals associated with respective first and second users.
In order to generate the two sets of vertical and horizontal polarization coefficients, the coefficient calculator is coupled to receive a set of channel fading coefficients from the channel estimator. Although not limited to any particular mechanism to calculate the channel fading coefficients, a non-limiting technique employs training sequences embedded in each user""s transmission burst. The vertical and horizontal signal inputs are correlated with both user""s (known) training sequences, to produce a set of peak values. Estimates of the channel fading coefficients may be derived by means of standard signal processing algorithms using the peak values and the known cross-correlation between training patterns. Alternatively, adaptive methods based on the received data or blind methods based on statistical properties may be employed.
The coefficient calculator computes the polarization coefficients required by the signal separator by means of a coefficient matrix such that, in the absence of noise, the output of the signal separator is equal to the user""s information signal. The condition the signal separator must satisfy for perfect signal separation is a set of four linear equations having four unknowns, a solution for which is determinable, provided that the fading coefficient matrix is full rank. When a statistical description of additive noise is available, the coefficient requirements may be modified in a manner that enables the coefficient calculator minimize the mean square error in the signal estimates.
As an alternative to using a pair of mutually orthogonally polarized antenna elements, it is also possible to employ plural sets (pairs) of antennas each comprising a pair of cross-polarized elements (horizontal and vertical). The inputs from these sets of cross-polarized antenna pairs can be weighted and combined upstream of the signal separator in order to optimize the two signals applied to its input ports.
In addition, if the channel is subject to frequency selective fading due to multipath, and the signals from different users have low cross-correlation properties, the frequency selective fading may be converted into a substantially flat fading channel by coherently combining the observed multipath. This coherent combining may be readily be implemented by means of a Rake receiver for each polarization installed upstream of the signal separator.